What is The PFM Flea? The PFM Flea is a very high-performance, low power voltage regulator optimised for (but not limited to!) powering an aftermarket clock for CD and DVD players: the TentLabs XO. This circuit is developed by a number of members of the Pink Fish Media forum.
For more extensive information: see here on the Pink Fish Media forum and on www.acoustica.org.uk.
In a CD player, but also in other digital audio circuits, a stable masterclock signal is extremely important. It is the 'heartbeat' of the circuit, so to speak: the conversion of the digital signal to the analog output signal is controlled by it. Any small deviation in time could cause a sample to be converted a bit too early or a bit too late, which results in small errors in the analog output signal. This will then deviate from the value that was originally intended. The result of this is a decrease of the detail and space in the soundstage. This deviation in time of the clock signal is called jitter. It is important to generate a clock signal with the least possible amount of jitter. On the next page this subject is explained further.
For a stable clock signal, a good oscillator is required, which is powered by a low-noise power supply.
The Flea is such a supply. On the Flea's PCB, an oscillator module of the desired frequency is mounted and The Fea provides it with clean power. It is recommended to select a type that has good jitter specifications, like a Tentlabs XO.
The output voltage of The Flea has to be adapted to the circuit it is connected to. If the circuit's logic operates on 5V or 3.3V, The Flea has to be set to this voltage accordingly. On the next page, this subject is explained further. The output of The Flea can be connected to the PCB of the player with some thin coax or a piece of twisted wire. Often, the empty pads of the original crystal and/or parallel capacitor can be used once they have been removed.
The hart of this circuit is a very low-noise opamp (typ. 0.9nV/√Hz) from Analog Devices, the AD797. It is wired as a voltage regulator and by feedback through a voltage divider it keeps the output voltage at the desired level. This opamp can supply enough oputput current to feed an XO-module directly, so an external output transistor is not needed. The incoming supply voltage is extensively filtered and pre-regulated with a 7812. A green LED supplies the necessary reference voltage and this is passively filtered with an R-C filter before it is fed to the opamp.
The Flea needs a supply voltage of 18V minimum. This can be obtained from the CD-player, but a better solution is to feed the clock from the mains with its own power supply. The well-known recipe of transformer, recitier, cap and regulator is well suited for this. In this case, a modified ready-made module from Conrad (190-835) is used, and this supplies a regulated 18VDC to The Flea. By using a separate dedicated supply, the coupling of noise through the supply-lines is prevented as much as possible. In the end, a reasonably simple but very effective circuit can been obtained this way.
The Printed Circuit Board
Printed circuit boards of the DIY-version of this design can be obtained through this website. See Ray's Audioshop page for details.
The original version from Pink Fish Media
In practice, the AD797 seems to be hard to obtain sometimes (and rather expensive). As a possible alternative a few other types can be considered from Analog Devices (AD), but also from other manufacturers like Linear Technology (LT), National Semiconductor (NSC) and Texas Instruments (TI), which also produces the Burr Brown opamps. In general, these opamps have a low noise voltage, good PSSR and are able to supply at least around 20mA output current. This is sufficient for one XO-module. Capacitor C8 is only necessary for the AD797 and must be removed for the other opamps. Click on the typenumber in the table to see the datasheet, the prices are not up to date and only indicative.
Most opamps function without any problems in The Flea. However, there are a few exceptions. Unfortunately, the three LT types don't seem to work in The Flea: the output voltage doesn't come on. The large capacitive load at the output is probably the cause of this. With the two THS opamps from TI, oscillation can be seen on the output voltage. Although it has only a small amplitude and the output voltage comes on nicely, it is a clear sign of instability.
The AD8597 from AD, and the OPA211 and OPA1611 from TI are opamps with very good specifications and they work without any problems in The Flea. The price is a bit better, only these types are not available in a DIL housing, so an SMD adapter is needed. The LME49710 from NSC is the single version of the LM4562. Although a dual LME49720 also exists, the specifications are identical. This popular audio-opamp also works fine in The Flea. With the OPA227, the voltage also comes on without any problems. The noise of these last two opamp is a bit higher, but on the other hand, they are available in a DIL-package and are a good replacement for the real low-cost alternative: the NE5534A.
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